A polymer film (hereinafter referred to as TAC film) formed of cellulose ester, in particular, cellulose triacetate (hereinafter referred to as TAC) with average acetylation degree of 58.0% to 62.0% is used as a film support for a photographic photosensitive material owing to its toughness and fire-resistant property. Because of its excellent optical isotropy, the TAC film is used as a protection film for a polarizing filter in an LCD device whose market is expanding.
The TAC film is produced by a solution casting method. The solution casting method is capable of producing the film with superior physical properties such as optical property when compared to other film producing method such as a melt-extrusion method. In the solution casting method, a high polymer solution (hereinafter referred to as dope) in which a polymer is dissolved in a mixed solvent containing dichloromethane or methyl acetate as a main solvent is prepared. The dope is cast from a casting die onto a support to form a casting film. A casting bead is the dope between a discharge opening (a slit) and the support. Upon obtaining a self supporting property on the support, the casting film is peeled off from the support as a wet film. The peeled wet film is sent to a tenter device through a transfer section provided with a plurality of rollers. In the tenter device, the wet film is dried while being stretched in a width direction. Thus, a film is produced. Thereafter, the film is dried again and wound by a winding device.
At the casting of the dope, both side edge portions of the casting bead in the width direction are tend to dry up faster than the center portion of the casting bead. As a result, agglomeration that is semi-solidified dope in an icicle-like shape is frequently formed at each side edge portion of the discharge opening of the casting die, and skinning is frequently formed on the both side edge portions of the casting bead. Conventionally, to prevent drying of the casting bead, an anticoagulant composed of a solvent of a dope or the like is supplied through a liquid supplying nozzle (see U.S. Pat. No. 5,393,476 corresponding to Japanese Patent No. 2687260, and Japanese Patent Laid-Open Publication No. 2002-337173), or both side edge portions of the discharge opening of the casting die are formed in specific shapes (see Japanese Patent Laid-Open Publication No. 2002-103361).
On the other hand, in the case the casting is performed at a high speed, it becomes necessary to rotate an endless support (for example, a casting drum or a casting belt) at a high speed accordingly. As the film production speed increases, an air flow generated by the rotation of the support becomes a serious problem. Such air flow is one of important factors which cause vibrations (flapping) of the casting bead. To eliminate the influence of the air flow to the casting bead, a decompression chamber is disposed in an upstream direction from the casting bead with respect to the moving direction of the support to reduce the air flow. The vibrations of the casting bead caused by the above air flow are reduced by supplying a solvent similar to the above anticoagulant to the both side edge portions of the casting bead (see Japanese Patent Laid-Open Publication No. 2003-181857).
In order to further improve production efficiency of the polymer film, it is effective to increase the casting speed and the width of the film. Additionally, the market demands a thinner and lighter LCD, which requires a thinner polymer film used as a protection film for the LCD. However, increasing the casting width and thinning the casting bead result in spattering of the anticoagulant due to the suction air of the decompression chamber under high decompression when the anticoagulant is supplied to each side edge portion of the casting bead only by using a liquid supply nozzle as in the conventional methods. Therefore, it has been difficult to supply the anticoagulant to an appropriate location, and skinning and the agglomeration have been frequently formed at the both side edge portions of the casting bead and the discharge opening.